1. Field of the Invention
The present invention relates to an aberration correcting apparatus for an information recording medium play-back apparatus. The information recording medium is generally an optical disc or similar medium.
2. Description of the Prior Art
In recent years the digital video (or versatile) disc (DVD) has attracted much attention as a large capacity digital information storage medium. Digital information such as animation, sound, computer information, or the like can be recorded on an optical disc having the same diameter, 12 cm, as a compact disk (CD) but with a recording density 6-8 times larger than the CD.
The DVD employs a smaller track pitch than the CD. The smaller track pitch increases the recording density and when coupled with a reduction in pit size a high density recording of approximately 5 G-bytes per side of the DVD has been achieved.
To read the high density recorded information of the DVD, a spot diameter of a laser beam employed to read the optical disk, is reduced to a size less than the laser beam spot diameter used for the CD. To produce a laser beam spot with a smaller diameter for the DVD, the wavelength of the laser beam source is set to 650 nm, which is shorter than the 780 nm laser beam wavelength used for play-back of the CD. The numerical aperture of an objective lens of the DVD play-back apparatus is set to 0.6, which is larger than the 0.45 aperture used in the play-back of the CD.
Central to the operation of the high-density recording disc is the concept of a tilt-angle. A tilt-angle is defined as the angle of deviation from a perpendicular surface of the optical disc with respect to the optical axis of the optical pickup. The tilt-angle causes a wave front aberration, mainly coma aberration, resulting in a deterioration of the S/N ratio of the reproduce signal due to the aberration (tilt-angle). The tilt-angle may result from one of several sources, such as warping of the surface of the optical disc or a tilting of the optical apparatus. As a result of the increase in the numerical aperture, the amount of the tilt-angle allowed is decreased.
The laser beam spot diameter is proportional to the wavelength of the laser beam source and inversely proportional to the numerical aperture of the objective lens. Therefore, by increasing the numerical aperture without changing the wavelength of the laser beam source, the recording density can be increased. As a result of increasing the numerical aperture, the angle that the surface of the optical disc is allowed to deviate from the perpendicular, with respect to the optical axis of an optical pickup, is reduced.
The optical discs, such as the DVD and CD, are slightly warped due to their own weight or to manufacturing conditions. However, if an optical disc has a large warped surface its tilt-angle may be large and exceed a tolerable level so that the S/N ratio deteriorates and, therefore, an excellent reproduction signal cannot be obtained.
Accordingly, the present invention is directed to an aberration correcting apparatus and information recording medium play-back apparatus that is capable of correcting wave front aberration caused by the tilt-angle of an information recording medium.
According to one aspect of the present invention, there is provided an aberration correcting apparatus for correcting an aberration of a laser beam that is emitted from a light source and focused on an information recording medium by an optical system, the aberration correcting apparatus includes a detecting means for detecting a tilt-angle of the information recording medium, a wave front aberration correcting means aligned in an optical path between the light source and the information recording medium for correcting wave front aberration of an incident beam by changing a refractive index corresponding to an applied voltage, and a control means for controlling the wave front aberration correcting means based on an output of the detecting means.
In another aspect of the present invention, there is provided an aberration correcting apparatus for correcting an aberration of a laser beam that is emitted from a light source and focused on an information recording medium by an optical system, the aberration correcting apparatus includes a liquid crystal element aligned in an optical path between the light source and the information recording medium for correcting wave front aberration of incident laser beam, a first detecting means for detecting a tilt-angle of the information recording medium in a radial direction, a second detecting means for detecting a tilt-angle of the information recording medium in a tangential direction; and a liquid crystal element control means for outputting a first control voltage for correcting an aberration due to the tilt-angle in the radial direction and a second control voltage for correcting an aberration due to the tilt-angle in the tangential direction according to outputs of the first and second detecting means, wherein the liquid crystal element includes a first electrode for applying the first control voltage and a second electrode for applying the second control voltage.
According to the present invention, wave front aberration caused by the tilt-angle of an information recording medium can be corrected by a simple control.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.